Question regarding sins of the tongue

I’ve read a book from Fr. Belet that states,

"The person who maliciously robs his neighbor’s reputation is held to restoring it on the same level as someone who steals. If what you said is secret even though it be true, you are obliged to restore his reputation. Otherwise you will not go to heaven.
“But how can I restore it? you may ask. You must tell everyone present when you spoke ill not to believe you, that you spoke out of wickedness. If the person you defamed knows about it you are duty bound to ask his forgiveness, etc. Many have been damned for such defamations because words pass and we forget having said them; they make no scruples over them and never think of confessing them.”

Now, I’ve been under the impression that all sins committed but forgotten can be forgiven in a regular confession (even if forgotten and not mentioned specifically) so how can this book say a person can be damned?

Restitution is just, so we should try to make a habit of it. It will obtain for us an increase in grace, and it will remove certain impediments to contrition.

Is restitution necessary for forgiveness? Not absolutely. It is not always possible, or practical, to make restitution to those we have wronged.

To keep things simple, I look at it this way: confess your sins to a priest and do the penance that they recommend; this ensures forgiveness (if we are contrite). If you have any doubts, ask your confessor, and follow their directions.

St. Alphonsus writes:
Alvarez said, “Even if the spiritual Father should err, the obedient soul is secure from error, because it rests on the judgment of him whom God has given it as a superior.” And F. Nieremberg writes to the same effect: “Let the soul obey the confessor; and then, although the thing itself were matter of fault, he does not sin who does it with the intention of obeying him who holds to him the place of God, persuading himself (as is, indeed, the case) that he is bound to obey him;”

Here is a relevant answer in the “Ask and Apologist” section:

Thank you very much!

I’m not sure on this one. I see the point…but. I had a co-worker that constantly told lies about me to make me look bad in front of our boss. Later he got arrested for a crime he committed on his personal time, and he’s in jail. Sometimes I pray for him, and while I was angry for a while with him, I’ve forgiven him now and wouldn’t want him to go to hell. I doubt he’ll ever admit what he did to my face, or apologize, or even admit to my boss that he lied about all that stuff but I don’t need him to in order to forgive him. Him apologizing to me wouldn’t have any impact, I don’t think, on how I feel.

But maybe this is about making things right with God, rather than with the victim?

Without getting into that book - I do not know it…

Talk with your confessor with any questions.

About forgotten mortal sins:

If one forgets a mortal sin but is repentant -seeking to confess all mortal sins in confession that forgotten mortal sin is* absolved *indirectly.

If one remembers a mortal sin one committed but forgot- one is to mention it in the next confession.

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